Most devices present multiple interfaces via SNMP, which can sometimes be confusing. A good example of this is Apple's Airport / Time Capsules.

If you're unsure if the interface you've chosen is the right one, a good test is to use the Configuration Assistant's Interface selection view to watch all interfaces simultaneously whilst generating a known amount of traffic (speedtest.net is again useful here).

This technique will help to narrow down which interface(s) are the most appropriate.